Re: Synchronization: PPS and Tx signal in a USRP N200

[jmfriedt]

If it can be of any help, we discussed generating a GPS-aligned 1-PPS
in http://jmfriedt.free.fr/ifcs2021.pdf and the oral presentation at
http://jmfriedt.free.fr/ifcs2021_presentation_jmfriedt.mp4 prepared
for the International Frequency Control Symposium (IFCS).

What we realized when working on this topic is that the only place
where time is known in an SDR processing sequence is at the ADC sample
acquisition step, since all subsequent processing and data transfer
afterwards are asynchronous, including the multitasking non-real time
operating system GNU Radio is running on. Hence, the control on the
1-PPS alignement can only be achieved at the FPGA level clocking the
ADC: the host clock is irrelevant since we do not know how much time
was needed to transfer and process data. Luckily, GNU Radio provides a
timestamp on each sample, assuming no data was lost since streaming
started, so the control signal can be fed back to the SDR clock if the
delay between reference and generated 1-PPS is measured using the same
time reference of the ADC clock.

Best, JM

> > On 2021-12-10 17:25, isaac mario tupac davila wrote:  
> > > Hello everyone!
> > >
> > > My name is Isaac. I have a curious situation here... I've
> > > generated a pulsed signal per second. I've saved one second
> > > period in a .h5 file, so that I'm sure I'm having a fixed signal
> > > per second, and then repeat it in my GRC flowgraph. .
> > >
> > > To use this signal as Tx I'm trying to synchronize a USRP N200 and
> > > a trimble in GRC. After run my flowgraph, this is what I see:
> > >
> > > imagen.png
> > >
> > > imagen.png
> > >
> > > The yellow signals are the PPS of my trimble and my purple signals
> > > are my tx signal per second. They are periodic in time but when
> > > you see it deeper, the difference between the timble PPS and my Tx
> > > signal is approx 97 ms. I think this difference should be close to
> > > cero, as my USRP and trimble are synchronized. ¿What is happening
> > > here? ¿Is this a normal behavior? I'm using unknown PPS to
> > > configure my UHD:USRP sink in GRC....
> > >
> > > I'll appreciate any help to clarify this behavior
> > >
> > > Regards
> > > Isaac T.
> > >  
> > Your flow-graph made very little sense to me.  If you want to 
> > synchronize your TX, you have to take explicit measures to ASK the
> > USRP to schedule your transmits at specific times.
> >    The 1PPS signal only synchronizes an internal time-stamp clock in
> > the unit.  It has NO WAY of knowing what the *meaning* of your
> > samples are, so it can't possibly
> >    synchronize some arbitrary event in your continuous sample stream
> > to 1PPS without you explicitly asking for when your bursts need to
> > be sent.
> > 
> > https://kb.ettus.com/Synchronizing_USRP_Events_Using_Timed_Commands_in_UHD
> >   
> 
> 
> 



-- 
JM Friedt, FEMTO-ST Time & Frequency, 26 rue de l'Epitaphe, 25000
Besancon, France